~ Stuff I Learned About Tech+Hydroponics

Getting Quicker Turn Around on Dev Boards

D’oh moments. The D’oh moments mean a two-three week turn around time getting to a PCB that fixes these mistakes. Time is something we can never replace. While I feel I am making too many mistakes, I felt the added time between fixing had an additional unwanted benefit of letting me dwell in the mistake instead of taking the mindset of incremental improvement. It is a minute better spent being happy and inspired instead of depressed and defeated.

There are two changes in the PCB build process that I will implement going forward:

pursuing opportunities to carry on testing with a PCB that has a broken design by spending more time on bodging techniques.

Break out the PCB into testable circuits and build PCBs with the smaller circuits using a CNC machine.

This post focuses on the second opportunity.

The opportunity is to imagine and explore a solution that is closer to the more forgiving world of software development where a mistake is quickly healed by changing code and recompiling. This post is a path I took to explore this opportunity. I went from layout to board in less than 8 hours. This is without knowing how it is done – other than I have a bunch of Gerbers.

The Goal

The goal of this post is to describe a new process I will try to reduce the turn around time in changing the design and testing development PCBs. The time benefits would be significant. It takes 2-3 weeks from sending the Gerbers to Osh Park to receiving the purple envelope. With the holiday season approaching, there will be vacation days, bad weather, and large volumes of packages that will conspire to increase the turn around time.

Thanks to Those That Went Before

I will (and must) never forget the exceptional Contextual Electronics courses I took from Chris Gammell. His ongoing mentoring has been instrumental in me going from knowing next to nothing about electronics a year ago to building PCBs whose function I have a passion about (hydroponics). Chris never turns “the bozo bit” – never assuming I can’t get a concept. Rather he helps guide me to a mental model in which exciting AHA! moments happen. It is true (and unfortunate for Chris) that it turns out I am not the sharpest tack in the drawer. But I am probably the most persistent.

Lauren at Metrix Create:Space walked me through the process of taking a set of Gerber files and building a PCB. I am grateful she took the time to explain the steps.

What a wonderful person working at a great place.

The Process

How to Make the PCB

I do not have a CNC machine and additional materials needed to fabricate a PCB. Even if I did, I do not have any experience. So I decided to support a local business. Our local Hacker Space, Metrix Create:Space, could create a “while you wait” PCB. So I thought I’d give their service a twirl.

Instead of just a CNC machine, they use both a CNC and a laser machine. The CNC drills the holes and cuts out the majority of the FP4.

Sketching the circuit with a laser lets the service get a much greater accuracy. I believe Lauren said down to 1 mil traces. The smallest trace I use is 10 mils. So, while it is super crazy to get down to 1 mil, this is lost on my prototyping needs.

Another Service Option

As I was exploring different places to make a test PCB, I stumbled across AP Circuits. They are located in Alberta, Canada. Their focus is on building and shipping prototype PCBs – which is pretty much what I’m after. Even with $31 for shipping, it is a less expensive option than what I chose. Yet it didn’t have the local turn around and fun factor of being part of the fabrication process – as well as immediate delivery.

I received a quote for their basic service:

Our In House Basic Service offers next business day shipping, however our Basic Service does not include Solder Masks/Silkscreens. If Solder Masks and/or Silk are required, our Plus Service can be offered. However our Plus Service ships in 3 business days. Please see pricing below:Circuit boards of 2.0″ x 2.2″ on FR4 .062″ (5.1 cm x 5.6 cm)

– Basic Service

– No solder masks, no legends

– 30 Drill Hits per Board

– 1 Oz Copper- IR Tin Lead Finish

Ships next business day.Quantity: 2, Price: $39.14 US*

Quantity: 6, Price: $84.06 US*

Quantity: 10, Price: $121.50 US**Orders are in even quantities only.

*Shipping and handling charges not included but are typically $31.00 for most North American destinations.*Applicable taxes not included.

Design of Circuits for Test PCBs

The design and layout of the Healthy Shields is too big and complicated to work well for a test PCB. The sub-circuits of the Healthy Shields include:

Power

Pumps

Temperature

Analog pH voltage measurement

Analog EC voltage measurement

analog – digital converter

communication to/from an Arduino (includes firmware)

I have already tested the power and pump circuits. I decided I will build test PCBs about 1” x 1” . The size of a shield is 2.1” x 2.7”. The smaller size will be easier and cheaper to build. The test PCBs I will build include:

Temperature and Analog pH voltage measurement. How close are the measurements to expected values? How noisy are the measurements? Of course, noise is dependent on the power source. I will be using my bench regulated power supply. An area of noise I wish to study in more detail is potentially contributed by VGND.

Analog EC voltage measurement. Same questions apply.

analog-digital converter. I’m moving off the MCP3901, replacing with the MCP3221. The main reason is it’s way too sophisticated for the project. Using the MCP3901 is like

driving a jet to a destination when all that is needed is

something to get the pH values to the Arduino with minimum complexity. The MCP3221 is I2C based which is a simpler interface than SPI. It has one channel and a simple way to convert from analog to digital.

Layout

The layout for each test PCB needs to be done. This is a very time consuming process that reminds me of putting together a puzzle with many small indistinguishable pieces. The biggest challenge were vias. I have been liberally spreading vias over the layout so that ground planes are on the bottom layer and paths to the ground planes from the top layer are handled by vias. The via size is pretty small at 30 mils. Osh Park’s service easily handle vias. The test fabrication setups can drill holes for the vias – after all, they use a CNC machine. But the setup does not include copper plating. When building test PCBs, I will minimize the number of vias and also increase the via size such that the drill size is 60 mils. It is a large drill size. But it will allow me to use 24 AWG wire to solder both sides to. What can I say? I am still on training wheels when it comes to my electronics skills.

The Cost

loaded the Gerbers into the PC software, changed the drills on the CNC machine as needed, made sure the CNC and laser machines ran properly, got the cut PCB ready for the laser sketching, ran the software so the laser sketched out the circuit,

Oooh – View from a microscope

and cleaned up stray copper that didn’t get cut off by the laser.

Machine and material time – Everything is supplied. The FP4, the machines, the tools…

Output of CNC Machine

Laser

Total Cost:

What’s Next

I’m spinning out the test PCB design/layout/building. Along the way, I am making mistakes which causes me to adjust and start again. I’ll get better as I go through the process. Yes, the expense adds up…